The chemical treatment of hydrocarbons and other compounds in the petroleum refining industry involves passing a fluid including both vapor phase and liquid phase components through a catalytic agent in a reactor to remove contaminants such as iron, other metals, nitrogen, sulfur, and calcium from the fluid. Examples of such processes in the petroleum refining industry include, for example, catalytic dewaxing, hydrotreating, hydrodesulfurization, hydrofinishing, hydrocracking, denitrogenation, hydrodenitrogenation combinations thereof, and others. These reactions are carried out by contact of a hydrogen-containing gas with a hydrocarbon feed stream at elevated temperatures and pressures in a hydroprocessing catalyst of a reactor.
One type of catalytic reactor system called a down-flow reactor includes one or more stationary catalyst bed through which a reactant stream of liquid hydrocarbon and hydrogen flows in a downward direction. As the reactant stream flows through the catalyst bed the liquid and vapor phases of the stream have a natural tendency to segregate and find separate paths through the catalyst. This tendency for the hydrogen and the hydrocarbon to find separate paths is commonly known as "channeling" and is highly detrimental to the overall efficiency of the reactor. The channeling causes formation of localized hot spots in the catalyst bed, reducing catalyst efficiency and requiring the catalyst to be replaced more often. The down time required for replacement of catalyst reduces the throughput of the reactor.
The channeling effect is increased by some of the chemical reactions which take place in these processes which may produce additional components in the vapor phase. In addition, because these reactions may also consume some of the hydrogen, it is frequently necessary to add additional hydrogen at various points within the reactor. When these vapor phase components separate from the reactant stream, are produced, or are added to the reactor, it is important that they be mixed with the liquid phase hydrocarbon components passing through the catalyst. To achieve the desired mixing of liquid and vapor phase components, mixing chambers or mixing boxes are positioned within the reactor at locations between successive catalyst beds.
The mixing chambers are used to uniformly redistribute the liquid and vapor phases at one or more locations within the reactor. This mixing counteracts the natural tendency for the liquid and vapor phases of the feed stream to segregate as the stream passes through the catalyst and seek separate paths through the catalyst. By providing a uniform distribution of liquid and gas, the catalyst will be efficiently utilized and the desired catalytic reactions will take place in a more predictable manner.
Further, radial thermal gradients, or hot spots, often occur in the catalyst beds of a reactor when the temperature of the fluid entering the catalyst bed is non-uniform. For example, quench gases may be added at one or more location when necessary to cool the reactor. Failure to fully mix the quench gases with the reactant stream, may create a temperature gradient across the reactor. The existence of hot spots within the catalyst bed leads to indiscriminate or non-selective hydrocracking of the hydrocarbons. For this additional reason, it is important to have mixing chambers within the reactor which provide a consistent fluid temperature distribution across the reactor. The better the mixing provided by the mixing chambers, the better the temperature and reaction control and the overall reactor performance.
Various systems have been proposed for achieving mixing in down-flow, catalytic reactors, for example, U.S. Pat. No. 3,541,000 includes a distributor employing liquid downcomers which allow the reactant stream collected on a collection plate to overflow into the downcomers. Angled chutes at the bottom of each of the downcomers cause the fluid to rotate within a mixing chamber beneath the collection plate. The rotation of the fluid promotes mixing of liquids and vapors.
Other distributor chambers for multi-bed, down-flow reactors are described in U.S. Pat. Nos. 5,462,719; 4,836,989; 4,182,741; and 3,977,834. However, many of the distributor chambers currently in use create an undesirably large pressure drop and/or provide incomplete mixing of components.